1. Field of the Invention
The present invention provides apparatus for measuring and recording systolic and diastolic blood pressure by an indirect auscultation method, and particularly provides for long-term ambulatory monitoring of blood pressure by recording it on tape and subsequently playing it back from the tape at a higher speed for analysis and plotting.
2. The Prior Art
The auscultation method dates from the discovery in 1905 by Korotkow that sounds audible on a stethoscope are produced when the flow of blood through a brachial artery is partially obstructed. According to that method, an inflatable cuff is placed on the arm and inflated to a pressure greater than the systolic blood pressure of the patient. Thereafter, the pressure in the cuff is gradually reduced and at first no sounds are heard. When the decreasing pressure in the cuff has fallen below the systolic pressure, Korotkow sounds are produced, one such sound being produced for each beat of the heart. As the cuff pressure decreases further, approaching the diastolic pressure, the Korotkow sounds continue to be heard, but as the cuff pressure falls below the diastolic blood pressure, the Korotkow sounds are heard no more, being inaudible for cuff pressures less than the diastolic blood pressure.
Considerable effort has been applied to automate the auscultatory method of measuring blood pressure, so that measurements can be performed by relatively unskilled personnel, or automatically, once the apparatus has been set up. A persistent problem has been the discrimination of the Korotkow sounds from other acoustical signals referred to in the art as artifacts. These artifacts, or false signals, can be generated by muscular movements of the patient among other things, and if they are mistaken for true Korotkow sounds, they will result in erroneous identification of the systolic or diastolic blood pressure. Thus, the artifact problem is especially critical when the device is intended for ambulatory monitoring of blood pressure, particularly in the case of children or disturbed persons.
From the standpoint of the comfort and safety of the patient, it is desirable that the duration of the measuring period as well as the cuff pressure be minimized. From the standpoint of the doctor, it is desirable that the results of the monitoring be available promptly and with a minimum of analysis, yet be in a meaningful form. These desiderata have been given due consideration in the present invention, which can best be appreciated in view of the prior art which will now be described:
In U.S. Pat. No. 2,827,040, Gilford discloses an automatic sphygmomanometer. Gilford uses a thermistor pressure impulse detector to sense the heart beat. The output of the pressure impulse detector is used to establish a time interval within which the K-sounds must occur to be regarded as true K-sounds. Gilford also requires two successive K-sound detections before a recording is made. Pressure is measured while the cuff pressure is increased, so that the diastolic measurement is made first and the systolic measurement is made last.
In U.S. Pat. No. 3,051,165, Kompelien uses an optical transducer applied to the patient's earlobe to sense variations in the opacity of the ear tissue to generate a signal from which the blood pressure can be detected.
In U.S. Pat. Nos. 3,996,926 and 3,893,452, Birnbaum discloses an invasive (catheter) blood pressure monitoring system and display.
In U.S. Pat. No. 3,137,292, Richter et al. determine the cuff pressure at the first and last K-sound in an automatic measuring cycle.
In U.S. Pat. No. 3,326,230, Follett generates narrow pressure calibration pulses as the steadily decreasing pressure drops below successive discrete pressure levels. These calibration pulses are superimposed on the K-sound signal and the combined signal is plotted. The systolic and diastolic blood pressures are then determined by inspection.
In U.S. Pat. Nos. 3,202,148 and 3,319,623, London shows apparatus for monitoring blood pressure using a column of indicator lights corresponding to successive pressure levels. As the cuff pressure decreases, a light is lit corresponding to the instantaneous pressure if a K-sound is present. At the end of a measuring cycle, the column will include normally several lights at the top or high pressure end which have not been lit followed by a series of lights which have been activated and followed by a few more lights which have not been lit. The uppermost and lowermost lights which have been activated indicate the systolic and diastolic pressure intervals respectively. In one version of the London apparatus, a paper chart plot is prepared automatically to show the systolic and diastolic pressures versus time.
In U.S. Pat. No. 3,450,131, Vogt uses a 1,000 Hz filter to detect artifact signals which are then used to establish discrimination gates for the K-signals.
In U.S. Pat. No. 3,654,915, assigned to the assignee of the present invention, Sanctuary shows an apparatus for automatically measuring and indicating blood pressure. The apparatus incorporates a number of advantageous safety features. The signal processing to eliminate artifacts includes the elimination of K-sounds which are neither preceded nor succeeded by another K-sound. Pressure is sensed by a mercury column manometer having a plurality of feeler contacts. The measurements are indicated by a column of lamps and can be recorded by a plotter. During each cycle of operation, the cuff is initially pressurized to a chosen level and thereafter the pressure is reduced through a linear cuff leak valve which is a distinct part from the dump valve used to vent the cuff pressure at the end of each measuring cycle.
The present invention differs from the invention of Sanctuary in a number of ways, which will become clearer below, but which will be enumerated here for convenience. The Sanctuary invention was only semi-portable, was power line operated and was not intended for long-term monitoring. In contrast, the present invention is truly portable and is intended for ambulatory recording during intervals up to twenty-six hours or more.
The Sanctuary invention included a column of lamps to display the successive measurements, although a paper tape record could also be provided. In contrast, the present invention has available an instantaneous digital display of each blood pressure result. In the present invention, the measurements are recorded on a magnetic tape and later replayed at high speed for plotting.
The Sanctuary invention used a mercury column manometer while the present invention uses a pressure transducer that operates on a different principle.
In the present invention, the same valve is used for bleeding the cuff and for quickly dumping the cuff pressure at the conclusion of the measuring cycle. In contrast, in the Sanctuary invention separate dump and bleeder valves are used.
More importantly, the present invention is distinguished from all of the above-mentioned prior art inventions, including the Sanctuary invention, in the following ways.
Firstly, in the present invention the blood pressure measurements are recorded on a magnetic tape along with simultaneously-made ECG (electrocardiogram) signals. This assures maintenance of the correlation of these signals with the blood pressure measurements and facilitates later analysis.
Secondly, in the present invention, the cuff pressure is stepped downward in discrete decrements triggered by successive ECG heartbeats or at discrete time intervals in the absence of ECG heartbeat signals. This has the advantage of speeding up the blood pressure measuring process over prior art methods. Some of the prior art inventions use cuff pressure bleeds which are linear or exponentially decreasing in time in an attempt to leak at a rate compatible with the slowest heart rate expected. Therefore, the leak rate is slow. The common method practiced by most physicians with simple apparatus, manually sets a fixed leak which slows down and uses more time per unit of pressure decrease as the pressure becomes lower.
Thirdly, in the present invention the initial pressure to which the cuff is pumped during the second and successive cycles of operation is based on the systolic blood pressure measured in the immediately preceding cycle. This speeds up the measuring process and also reduces patient discomfort by avoiding unnecessary pressure levels.
Fourthly, the present system makes use of a common automatic plotting mechanism to mark the systolic and diastolic blood pressure readings at set intervals on a heart rate chart to facilitate recognition of trends.
Fifthly, in the present invention, the cuff pressure transducer is located inside the housing of the blood pressure measuring apparatus and is connected to the cuff by an air hose which is separate from the air hose used for inflating and deflating the cuff. This expedient is an improvement over connecting the pressure transducer hose to the inflation hose at a point within the housing near the valve, because the cuff serves as an air ballast to substantially isolate the pressure transducer from pressure transients generated by the opening and closing of the valve.